BIOL 1400 -- Lecture Outline 34
Suggested Reading: Chapter 28, Biology: Concepts and Connections

"The question of whether a computer can think is no more interesting than the question of whether a submarine can swim." -- E. W. Dijkstra

I. Control and communication systems in the human body

  1. Nervous system -- the "telephone network" (because it carries point-to-point signals to specific sites).
  2. Endocrine system -- the "radio broadcast" (because it broadcasts signals to all cells in the body that are equipped to receive them)
    1. A gland is any organ or structure that produces and secretes something (enzymes, saliva, sweat, hormones, whatever)
    2. Any gland may be either exocrine, endocrine, or both.
    3. Exocrine glands secrete their products out through a duct. Examples: tear glands, sweat glands, salivary glands, prostate gland.
    4. Endocrine glands secrete their products directly into the bloodstream, and don't have ducts. Examples: adrenal glands, pituitary gland.
      1. Endocrine glands produce "chemical messenger" molecules that can be carried everywhere in the body by the blood, and that potentially can affect a wide variety of organs at once. These are hormones.
      2. To work, a hormone molecule must bind with a special receptor protein that sits on and in the membrane of a target cell.
      3. This sets off a chain reaction of molecular events in the cell -- we needn't worry about the details, but basically a cell may be stimulated to grow, die, change shape, stop growing, divide, produce new products, etc. etc. etc. . . . by the activity of the hormone.
    5. Some glands work both ways. For example:
      1. The pancreas secretes digestive enzymes into the intestine through a duct (exocrine function) and secretes the hormone insulin directly into the blood (endocrine function).
      2. The testes secrete sperm and a small amount of fluid through a duct, the vas deferens (exocrine) and secrete testosterone, the male sex hormone, directly into the blood (endocrine).
II. Nerve cells
  1. The basic units of the nervous system are specialized cells called neurons. These can take on many shapes, but a fairly typical neuron looks like this:
    1. Radiating out from the cell body are many fine, filament-like dendrites. These receive signals from other neurons.
    2. Also extending out from the cell body is (usually) one long axon. This transmits a signal to other neurons.


      Neuron from the spinal cord of a cow. Note thick axon and finer, thinner dendrites.

  2. Transmission of a nerve impulse
    1. Nerve cell membranes contain a protein, the sodium-potassium pump.
      1. his is an active transport protein: it requires energy to operate, which it gets from breaking down (of course) ATP into ADP and Pi.
      2. It uses the energy from ATP to pump sodium ions (Na+) out of the cell, and potassium ions (K+) into the cell, at the same time.
      3. A second protein in the membrane allows potassium, and only potassium, to passively diffuse across the membrane.
      4. This leads to the membrane having a slight negative electrical charge inside, and a slight positive electrical charge outside. The difference between the two is very small, about -70 millivolts (thousandths of a volt).
      5. When a neuron is stimulated, a third protein in the membrane, normally closed, swings open and allows sodium to diffuse into the cell. (It's called a voltage- gated sodium channel, by the way.)
      6. For a brief moment, a small area of the cell membrane reverses its electrical polarity as the sodium ions rush in. So much Na+ rushes in, that the surrounding area of membrane becomes positively charged inside and negatively charged outside.
      7. Channels that let K+ flow out, and the Na+-K+ pump, eventually return the membrane to its normal state. But before that happens, closed sodium channels close to the area of reversed polarization are stimulated to open. . .
      8. . . . and a wave of electrical potential travels down the neuron. (Much like people "doing the wave," or like one of those electrical signs with many light bulbs that turn on and off in waves.)
      9. This wave is called an action potential.
        1. IMPORTANT: An action potential is an all-or-nothing thing. A neuron will either transmit a full-scale action potential, or it won't transmit one at all.
        2. You perceive the intensity of stimuli by variance in the rate, not the intensity, of action potentials. In other words: If you are touched, neurons in your skin will send action potentials all the way up to your brain. If it's a light touch, they might only fire a few per second; if it's a sock in the jaw, they might fire thousands of times per second. But the action potentials themselves are always of equal strength.
    2. What happens when the action potential reaches the end of the neuron?
      1. The point where an axon of one neuron and a dendrite of another neuron meet is called a synapse.
      2. The two neurons don't actually touch at the synapse. There's a minuscule gap in between the two neurons.
      3. Inside the tip of a neuron's axon are a great many vesicles. Each is filled with small molecules called neurotransmitters.
      4. When an action potential hits the end of an axon, the vesicles break open into the synapse, and the neurotransmitter molecules spill out into the synapse.
      5. The neurotransmitters bind to receptor proteins on the other side of the synapse. This stimulates an action potential to start moving down the second neuron.


        Cross-section through a synapse. You can see the vesicles breaking open at the cell membrane, releasing their contents.

      6. When neurotransmitters have been released into the synapse and have done their job, they are either broken down by special enzymes, or taken back into the cell that they came from.
        1. If neurotransmitters weren't broken down or taken up, they would continue to stimulate the "downstream" neuron over and over.
        2. Many drugs, legal and otherwise, work by blocking this re-uptake of neurotransmitters. EXAMPLES:
          • Prozac blocks the re-uptake of the brain neurotransmitter serotonin.
          • Cocaine blocks the re-uptake of the brain neurotransmitter dopamine.
          • Sarin (nerve gas) blocks the re-uptake of the transmitter acetylcholine, which stimulates muscles to contract.
        3. Other drugs, legal and otherwise, work by mimicking a neurotransmitter.
          • LSD mimics the brain neurotransmitter serotonin.
          • Psilocybin (the active ingredient in "magic mushrooms") and mescaline (the active ingredient in peyote cactus) mimic the brain neurotransmitters dopamine and/or noradrenaline.

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